This invention relates to a circuit arrangement for supplying a discharge lamp with a direct current, provided with
input terminals for connection to a supply voltage source, PA1 a first branch comprising capacitive means and interconnecting the output terminals, PA1 a second branch comprising a series arrangement of second capacitive means and an impedance and shunting the first branch, and PA1 a first terminal for holding the discharge lamp connected to a first end of the series arrangement and a second terminal for holding the discharge lamp connected to a second end of the series arrangement.
a circuit portion I coupled to the input terminals for generating a DC voltage from the supply voltage delivered by the supply voltage source, PA2 said circuit portion being provided with output terminals between which the DC voltage is present during operation,
Such a circuit arrangement is known from patent document U.S. Pat. No. 5,343,125. The impedance present in the second branch is formed by an ohmic resistor in the known circuit arrangement. The first capacitive means serve as an output buffer capacitor. Immediately after ignition of a discharge lamp connected to the known circuit arrangement, the discharge is not yet stable and the impedance of the discharge lamp is comparatively low. The first capacitive means deliver a first current having a comparatively high amplitude to the load during a comparatively short time interval immediately after ignition. At the same time, the second capacitive means deliver a second current to the load. However, since the amplitude of the second current is limited by the impedance formed by the ohmic resistor and present in the second branch, the second current will flow during a considerably longer time interval. Given a suitable dimensioning of the known circuit arrangement, the second current has a sufficiently high amplitude and also flows long enough for maintaining the discharge in the discharge lamp immediately after ignition for so long as is necessary for this discharge to stabilize itself. It is thus achieved in the circuit arrangement that a discharge lamp operated by means of the circuit arrangement has a good ignition behavior. The voltage across the discharge lamp is comparatively low while the discharge is stabilizing during a first time interval immediately after ignition. During the last stage of the stabilizing process of the discharge, however, the voltage across the lamp rises to the value belonging to stationary lamp operation. An important disadvantage of the known circuit arrangement however, is that a considerable portion of the current supplied by the second capacitive means does not flow to the discharge lamp but to the first capacitive means when the voltage across the discharge lamp rises during this last stage. The current supplied by the second capacitive means flows both through the discharge lamp and through the first capacitive means as the lamp voltage rises, which means that the second branch operates comparatively inefficiently in this phase of the lamp operation, which may even cause the discharge lamp to extinguish.